Attributes

Attributes are pure metadata: they flip flags the compiler reads. They never generate code, transform the AST, or run user logic.

#[test]: register a test function

#[test]
fn it_adds() {
    let r: i32 = add(2, 3);
    assert r == 5;
}

Run with cpc test. The assert intrinsic sets a failure flag in a test build and traps in a regular build.

#[repr(C)] and #[link_name = "..."]

Layout and symbol control for FFI. Both are covered on the FFI page: #[repr(C)] promises platform C-ABI layout for a struct that crosses an extern fn boundary, and #[link_name] binds one C symbol under several typed shapes.

Loop hints: #[unroll(N)] and #[vectorize_width(N)]

Statement-level attributes that flow through to LLVM's loop optimizer as !llvm.loop metadata. Apply them to a while, loop, or for. N is a literal in [1, 256].

#[unroll(4)]
while i < n {
    sum = sum + buf[i as usize];
    i = i +% 1;
}

#[vectorize_width(8)]
for i in 0..count {
    out[i as usize] = a[i as usize] * b[i as usize];
}

They are marginal for general code but load-bearing for the tight inner loops the compiler does not vectorize well by default.

#[inline]: inlining control

A function- or method-level attribute, in three forms:

• #[inline] becomes LLVM inlinehint. Only the optimizer acts on it, so it matters at --release and is a no-op at debug -O0.
• #[inline(always)] becomes alwaysinline. It inlines even at -O0 and past LLVM's cost threshold, the lever for hot kernels built from small over-threshold helpers.
• #[inline(never)] becomes noinline. It pins a call boundary for cold paths or code-size control.

These are pure hints with no behavioural change, so reach for them only on measured hot or deliberately cold paths.

Real-time contracts

#[no_alloc], #[no_block], #[bounded_recursion], #[max_stack(N)], and the #[realtime] bundle are compiler-verified contracts, not optimizations. They are documented in full on the Real-time page.

Doc comments

/// Returns the square of x.
fn sq(x: i32) -> i32 { return x *% x; }

/// comments are doc comments, and cpc test picks up fenced code blocks inside them as doctests.